1. Field of the Invention
The present disclosure generally relates to a polishing head, a chemical mechanical polishing apparatus using this polishing head and a method for controlling a polishing profile with the chemical mechanical polishing apparatus, and, in particular, to a polishing head using zone control.
2. Description of the Related Art
In microstructures such as integrated circuits, a large number of elements such as transistors, capacitors and resistors are fabricated on a single substrate by depositing semiconductive, conductive and insulating material layers and patterning these layers by photolithography and etch techniques. Frequently, the problem arises that the patterning of a subsequent material layer is adversely affected by a pronounced topography of the previously formed material layers. Moreover, the fabrication of microstructures often requires the removal of excess material of a previously deposited material layer. For example, individual circuit elements may be electrically connected by means of metal lines that are embedded in a dielectric, thereby forming what is usually referred to as a metallization layer. In modern integrated circuits, a plurality of such metallization layers are typically stacked on top of each other to provide the required functionality. The repeated patterning of material layers, however, creates an increasingly non-planar surface topography, which may deteriorate subsequent patterning processes, especially for microstructures including features with minimum dimensions in the sub-micron range, as is the case for sophisticated integrated circuits.
Further, the demand for higher integration, higher clock frequencies and smaller power consumption in microprocessor technology lead to a new chip interconnection technology using copper instead of aluminum for chip wiring. Since copper is a better conductor than aluminum, chips using this technology may have smaller metal components, and use less energy to pass electricity through them. These effects lead to a high performance of the integrated circuits.
The transition from aluminum to copper required, however, significant developments in fabrication techniques. Since volatile copper compounds do not exist, copper cannot be patterned by photoresist masking and plasma etching, such that a new technology for patterning copper had to be developed, which is known as a copper damascene process. In this process, the underlying insulating layer is patterned with open trenches where the conductor should be filled in. A thick coating of copper that significantly overfills the trenches is deposited on the insulating layer. The excess copper is then removed down to the top level of the trench. Currently, there is no effective copper dry etching method because of problems removing low volatility copper compounds. Presently, chemical mechanical polishing (CMP) is used for removing the excess copper.
The repeated patterning of material layers, however, creates a non-planar surface topography, which may deteriorate subsequent patterning processes, especially for microstructures including features with minimum dimensions in the sub-micron range, as is the case for sophisticated integrated circuits.
In conclusion, it is typically necessary to planarize the surface of the substrate between the formation of subsequent layers. A planar surface of the substrate is desirable for various reasons, one of them being the limited optical depth of the focus in photolithography which is used to pattern the material layers of microstructures.
Chemical mechanical polishing (CMP) is an appropriate and widely used process to remove excess material, including copper and tungsten, and to achieve global planarization of a substrate. In the CMP process, a wafer is mounted on an appropriately formed carrier, a so-called polishing head, and the carrier is moved relative to a polishing pad while the wafer is in contact with the polishing pad. A slurry is supplied to the polishing pad during the CMP process and contains a chemical compound reacting with the material or materials of the layer to be planarized by, for example, converting the material into an oxide, while the reaction product, such as the metal oxide, is then mechanically removed with abrasives contained in the slurry and/or the polishing pad. To obtain a required removal rate, while at the same time achieving a high degree of planarity of the layer, parameters and conditions of the CMP process must be appropriately chosen, thereby considering factors such as construction of the polishing pad, type of slurry, pressure applied to the wafer while moving relative to the polishing pad and the relative velocity between the wafer and the polishing pad. The removal rate further significantly depends on the temperature of the slurry, which in turn is significantly affected by the amount of friction created by the relative motion of the polishing pad and the wafer, the degree of saturation of the slurry with ablated particles and, in particular, the state of the polishing surface of the polishing pad.
FIG. 1 schematically shows a sketch of a conventional system 100 for chemical mechanical polishing. The system 100 comprises a platen 101 on which a polishing pad 102 is mounted. Frequently, polishing pads are formed of a cellular microstructure polymer material having numerous voids, such as polyurethane. A polishing head 130 comprises a body 104 and a substrate holder 105 for receiving and holding a substrate 103. The polishing head 130 is coupled to a drive assembly 106. The device 100 further comprises a slurry supply 112 and a pad conditioner (not shown).
In operation, the platen 101 rotates. The slurry supply 112 supplies slurry to a surface of the polishing pad 102 where it is dispensed by centrifugal forces. The slurry comprises a chemical compound reacting with the material or materials on the surface of the substrate 103. The reaction product is removed by abrasives contained in the slurry and/or the polishing pad 102. The polishing head 130, and thus also the substrate 103, is rotated by the drive assembly 106 in order to substantially compensate for the effects of different linear velocities of parts of the polishing pad 102 at different radii. In advanced systems 100, the rotating polishing head 130 is additionally moved across the polishing pad 102 to further optimize the relative motion between the substrate 103 and the polishing pad 102 and to maximize pad utilization. The pad conditioner may comprise an abrasive component, e.g., diamonds, embedded in a matrix. Thus, the surface of the polishing pad 102 is abraded and densified slurry, as well as particles that have been polished away from the surface of the substrate, are removed from voids in the porous polishing pad 102.
Various designs of chemical mechanical polishing devices are known in the art. For example, the rotating platen 101 may be replaced with a continuous belt kept in tension by rollers moving at high speed, or slurry may be injected through the polishing pad 102 in order to deliver slurry directly to the interface between the polishing pad 102 and the substrate 103.
State of the art CMP processes use polishing heads that are capable of adjusting the polishing removal rate to flatten the removal profile by applying zone dedicated back pressures to the wafers. This means that the polishing head can provide a non-homogeneous pressure distribution which allows control of the polishing profile such that the polishing profile may be adjusted to be inverse to a deposition profile of a preceding deposition process, e.g., with an electrochemical plating tool for copper deposition.
In metal polishing processes, specifically in a copper CMP and a tungsten CMP, newly developed polishing slurries tend to have less mechanical properties and instead more chemical polishing properties. That means that the polishing rate does not (strongly) follow Preston's equation (developed by Preston, 1927) which models the mechanical effects of pressure and velocity in the CMP process:R=K·P·V where R denotes the polish rate, P is the applied downward pressure, V is the linear velocity of the wafer relative to the polishing pad and K is a proportionality constant, called the Preston coefficient.
These slurries are specifically designed for very low down force processes that are typically employed with ULK materials (ultra low dielectric constant materials). Some of these slurries do not show any significant increase of the removal rate when the downward pressure is increased. As a result, the control of the removal profile is limited with conventional polishing heads using zone dedicated back pressures.
Therefore, one problem with conventional systems for chemical mechanical polishing is that control of the polishing profile is not sufficiently effective for CMP processes, in particular if ULK materials are involved.
The present disclosure is directed to various methods and devices that may avoid, or at least reduce, the effects of one or more of the problems identified above.